Present AC transmission lines, transformers, circuit breakers, and other devices, are connected in networks or grids. Such integrated networks, and their components, interconnect generating stations which supply electric power and substations which deliver electric power for distribution to consumers.
Presently these transmission facilities are used very ineffectively. Power transfers to or from one area or region to another are limited by loading conditions on a specific circuit(s) even though many other circuits having the capacity to deliver power are loaded below their ratings or capability. The resultant average loadings of transmission lines and equipment are significantly below their carrying capacities or ratings. In fact, studies show average loadings of about 30 percent of ratings on an annual basis and under 40 percent of ratings even during heavy loading periods. The improvement of the utilization of transmission facilities can produce benefits in two ways:
A. Make possible increased power transfers over facilities that have previously been installed. PA1 B. Reduce the need for transmission system additions required to meet future needs.
The present poor utilization of transmission results from a number of factors:
A. The major reason under steady state conditions is that the division of loadings on transmission lines and facilities in AC networks is in accordance with Kirchoff's Laws holding that the impedances of various network branches determine the loadings of the various branches. This results in a division of loadings which is not in proportion to the capacities or ratings. Also, electric power flow patterns are continually changing in transmission networks because of consumption changes and because the transmission and generation facilities in operation change from time to time.
In a simple example, if two electric power transmission lines of differing capacity and/or impedance connect two areas in a system, the steady state power transfer limit between the two areas with all facilities available is reached when the first line becomes loaded to its carrying capacity or rating. Since the second line is not loaded to capacity under this condition, additional power can be transferred over the two lines through the injection of of controlled circulating current using the means described herein to reduce the loading of the first line and increase the loading of the second line so that both lines can be loaded to their ratings.
B. When a transmission line or generator becomes unavailable for service due to a sudden failure and automatic disconnection by action of relays and circuit breakers, power flows are automatically redistributed in the networks in accordance with Kirchoff's Laws resulting in a new division of power flows which, in most cases, does not result in the loading of transmission lines and facilities in accordance with ratings. The rapid injection of a controlled circulating current(s) in one or more circuits of the network by the means described herein will redistribute the power flows as described in (a) above so that larger total power transfers can be safely made.
c. The schedules for electric power production among the generating stations in the network so as to meet total electric power requirements are varied as total power usage varies so as to optimize total incremental costs for generation considering the availability and cost of fuel. Such changes in schedules cause changes in loadings on the transmission systems resulting in transmission facilities not being loaded in proportion to ratings thus reducing transmission capability from one area to another. The rapid injection of a controlled circulating current(s) by the means described herein located in an appropriate circuit(s) will redistribute the power flows so that larger total power transfers can be made.
d. Sudden transient disturbances resulting from short-circuits or other failures in the transmission system produce oscillations in the instantaneous speed of generators with respect to the rest of the network which can cause them to lose synchronism requiring that they be automatically disconnected from service. Extra transmission facilities are sometimes provided to reduce such oscillations. Such oscillations result from an imbalance between mechanical power input to the generators and their electrical power output. The very high-speed injection of properly controlled circulating currents using the means described herein will change the pattern of transmission circuit loadings in the vicinity of such a generator so as to dampen such oscillations thus increasing safe generator loading limits for a given transmission system or reducing transmission requirements for future generator installations.
Dynamic oscillations can occur between areas or regions when interconnected with AC transmission which considerably reduce that amount of electric power that can be transferred. When the two areas that are to be interconnected are previously isolated from each other the oscillations may be of sufficient magnitude as to prevent an AC transmission tie and require the installation of a higher cost DC transmission tie. By properly controlled use of the means described herein in the transmission circuits interconnecting the two areas these oscillations can be dampened so that transmission capability is increased and the need for highcost DC transmission avoided.
e. AC transmission networks have circuit breakers installed at various points so that facilities on which short circuits occur are automatically disconnected. The circuit breakers, and associated bus equipment, must be able to withstand and interrupt the maximum short circuit current that may occur. As AC transmission networks grow and generating stations and substations are added, the magnitude of the short circuit currents can exceed the capability of the associated buses and circuit breakers to withstand and interrupt them, required a replacement of these circuit breakers and/or a physical replacement of the buses involved. By use of the means described herein, current may be injected at a sufficiently fast speed to reduce the resulting short circuit currents within the capability of the circuit breakers and buses, thus eliminating the need for expensive reinforcements and replacements.
By judiciously locating the device described herein in a circuit or number of circuits in the transmission network, a combination of the benefits described above can be obtained resulting in multiple benefits from each device.
In fact, to date a number of such methods exist for achieving some of the above improvements in the transmission of electrical power in AC networks. These have included:
1. Use of phase angle regulators or quadrature boosters. These are large, massive devices that are expensive. They have experienced frequent failures and cannot be changed in settings rapidly. However, such devices are being increasingly used in the U.S.A. and in other nations. They do not however have the ability to achieve the fast changes of the device described herein.
2. Use of DC transmission lines operating in parallel with the AC network. This, too, presents an expensive and complex solution. The DC lines require full capacity rectification and inversion equipment as well as a large amount of reactive correction capacity. The need to design the DC circuit and equipment for full circuit capacity, voltage, and insulation level results in higher costs compared to achieving the same results with an AC circuit equipped with the device described herein. Also, the reliability of such DC circuits to date has been lower than the reliability which has been achieved with AC circuits of similar capacity.
3. Use of operating personnel directly or through supervisory controls to adjust generation schedules at the generating stations or the electrical arrangement of a transmission network by closing or opening circuit breakers when this can improve the utilization of facilities under specific loading conditions. Such changes are usually achieved at a penalty in fuel costs and fuel consumption required to meet overall system energy requirements. Use of the device described herein will not require such fuel penalties.